In order to prevent counterfeiting of recorded articles such as for example printed articles produced by color copying technology using an optical image scanner and color printer or in order to ascertain genuineness by identifying whether a subject article is a forgery or a genuine article, recording or printing techniques are known in which special optical characteristics are applied to the surface of the printed article. Such recording techniques for preventing counterfeiting can be classified into two types, called overt and covert. The special optical characteristics of the overt technique are exposed i.e. can be identified simply by directly viewing with the naked eye under natural light. The special optical characteristics of the covert technique are hidden and can only be identified by viewing under special optical conditions or using special optical tools.
A typical example of an overt technique is recording or printing using a color-change ink; this is disclosed in Laid-open Japanese Patent Publication No. H8-324169 or Laid-open Japanese Patent Publication No. H9-254520 or Laid-open Japanese Patent Publication No. H10-060303. A color-change ink has the property that the main wavelength of reflected light therefrom when white light is directed on to the surface of the ink changes depending on the angle of incidence (angle of reflection). The perceived color of a surface printed with a color-change ink therefore changes when the angle of viewing is changed.
A typical example of a covert technique is recording or printing accompanied by surface processing to produce optical retroreflection properties, as disclosed in U.S. Pat. No. 2,407,608 or Laid-open Japanese Patent Publication No. H10-97208, for example. Optical retroreflection means a reflection characteristic whereby the angle of incidence and the angle of reflection are substantially the same i.e. a reflection characteristic in which incident light is bent back so that the reflected light is returned to the light source. A covertly recorded portion having optical retroreflection is of exactly the same color as the surrounding region when viewed under natural light and so cannot be distinguished, but can be distinguished from the surrounding region when viewed using a special tool.
Since both overt techniques and covert techniques require special inks or special processes for processing, a recorded article manufactured using such techniques cannot be reproduced using a simple color copier.
A recorded article to which an overt technique has been applied and a counterfeit article manufactured by simply copying this using a color copier can be differentiated visually simply by ordinary viewing under natural light. Accordingly, even supposing such a simple counterfeit product appears on the market, people in the market can easily notice that it is not a genuine product. The effectiveness of overt techniques in excluding simple counterfeit products from the market is therefore high. Other hand, since the location and type of such overt recording on the article in question are apparent to the eyes of anyone, this gives a clue to other persons intending to imitate such overt recording.
In contrast, the presence of covert recording cannot be noticed by people in the market. There is therefore little likelihood of clues to imitate this covert recording being given to other persons. However, people in the market cannot normally identify the presence of covert recording, so there is a high probability of people in the market being simply cheated even by simple counterfeit articles.